Glutathione excretion in response to heterologous protein secretion in Saccharomyces cerevisiae

Glutathione is excreted in a dose-dependent, non-stoichiometric fashion from Saccharomyces cerevisiae cells expressing and secreting Bovine Pancreatic Trypsin Inhibitor (BPTI), a small, disulfide-bonded protein. Glutathione excretion commences 40 hours following induction of BPTI synthesis. Expression of several secretory proteins with varying disulfide and cysteine contents results in glutathione excretion with no apparent requirement for protein disulfide content. Glutathione excretion is also triggered by overexpression of Kar2p/BiP, a native ER-resident protein-folding chaperone, indicating that the response is a general one not restricted to overexpression of thiol-containing heterologous proteins. Functional vesicular transport is not required at the time of glutathione excretion, and glutathione excretion requires the presence of molecular oxygen. These data are consistent with a delayed oxidative stress response potentiated by earlier heterologous secretion, but are inconsistent with secretory transport of glutathione spent as oxidizing equivalents for disulfide-bond formation in the endoplasmic reticulum.